We can over-stretch our metaphors.
Is a passport an “identifier”?
Is a drivers licence an identifier?
Is a credit card an identifier?
Is a professional membership card an identifier?
Is an employee badge an identifier?
Is a building access card an identifier?
Is a house key an identifier?
Is a car key an identifier?
Or putting the questions another way …
Is a car key a “key”?
Is a house key a key?
Is a building access card a key?
Is an employee badge a key?
Is a professional membership card a key [to access an association]?
Is a credit card a key [to a payments system]?
Is a drivers licence a key [to access the privileges of road usage]?
Is a passport a key [to enter another country]?
From When does a key become an identifier?, 28 April 2005.